This invention relates generally to apparatus and methods for cutting soil and constructing subsurface containment barriers in place. Although not necessarily limited to the following, this invention has particular application in simultaneously cutting through a subsurface volume of soil and emplacing a cement slurry to construct in situ a continuous subsurface wall, horizontal panel or basin around and under a hazardous waste site or contaminated land area. The invention generally may be applied in sites of wide-ranging size, from small sites with buried substances or objects that are too hazardous for excavation to large multi-acre sites having contaminated soils. Preferably, encapsulation may take place without drilling into contaminated soils.
There are many automated ways of cutting or excavating soil (soil as used herein refers to any ground or subsurface material to be cut or excavated). For example, there are scooping devices such as backhoes and clamshells; there are drilling devices such as augers; and there are blasting devices such as dynamite and high pressure fluid. Of particular interest to the present invention as used in the aforementioned exemplary application, however, are the devices and techniques used for cutting soil in the environmental remediation industry.
In the environmental remediation industry it is often desirable to form an impermeable underground containment wall to contain contaminants which are present in the soil and water, thereby preventing or impeding further migration of the contaminants. Hazardous waste sites frequently contain hundreds of thousands of cubic yards of materials which represent a long term threat to ground water quality. While on site treatment is a preferred means of eliminating this threat, this is not always feasible. At some sites the cost of physically removing the material and placing an impermeable liner in the vacated cavity is beyond the resources of the site owner. Sites with buried drums, radioactive dusts, or other airborne hazards may become much more dangerous if excavated. There are also cases where vast and deep areas are only slightly contaminated and require only a containment action. Existing containment technologies provide the means to place a wall around the perimeter of a site or to place a cap over a site.
One common method of constructing a side containment wall is by slurry trenching. This method digs a trench and emplaces a bentonite (clay) slurry as the trenching proceeds. Once the trench is dug, the slurry is replaced with concrete or bentonite modified clay. This technique tends to be slow and very costly at depths exceeding 40 feet. This technique is also limited to forming a relatively wide (e.g., 36 inches) wall even though it is only the thin filter cake build up on the wall that acts as a permeability barrier. The difficulties and expense of forming and ensuring that a continuous wall has been formed increases dramatically below a 40 foot depth, a depth below which this type of wall often needs to extend.
Hydraulic soil cutting using jet grouting is another technique used in the environmental remediation industry. Although this is a useful technique, it is not particularly efficient because much of the jet energy is wasted in passing through fluid before impacting the soil. This causes low production rates, and the cost of the process tends to be higher than for mechanical methods. In most forms of jet grouting it is also difficult to verify that a continuous wall has been formed because the wall is formed from a series of overlapping columns rather than in a continuous fashion. This makes it difficult to form containment walls deeper than 40 feet using this technique.
For forming deeper walls, a four-auger drill system and a clamshell digging tool have been used. The four-auger system is very expensive and slow, capable of forming only 20 to 30 linear feet of wall per day. Clamshell excavating techniques are also very slow.
The foregoing techniques typically provide vertical walls. They do not typically provide bottom barriers under the site, but rather they rely on having a natural layer of low permeability soil (e.g., impermeable rock or clay) underlying the waste site to complete the containment envelope. We are, however, aware of two prior ways of creating an underlying barrier.
Jet grouting technology as practiced by Halliburton Services of Duncan, Okla. allows a bottom to be installed by drilling vertical holes and using the jet grouting process to form overlapping disks of treated material at the bottom elevation. Just as with side wall jet grouting referred to above, it is difficult to verify the integrity of the resulting underlying barrier. Another technique uses horizontal drilled holes with liquid nitrogen freezing. This has quality control problems and requires continuous maintenance. Near surface horizontal pancake fracturing or "block heaving" is another technique which seems to work, but it is difficult to control quality with this technique.
For very large sites containing enormous volumes of waste such as are found in the mining industry for example, the primary, if not the only, suitable technique of waste containment of which we are aware is to physically move the waste onto a synthetic liner and place a cap over it. This has detrimental cost and environmental impact shortcomings as referred to above.
Although the foregoing techniques may be effective in particular applications, they have at least the shortcomings noted above. What is lacking is a cost effective technique for cutting soil to facilitate at least the deep construction of contaminated soil impoundment walls and subsurface containment barriers having high structural integrity around and under waste sites without moving the waste.